The paper traces how the American linguist William Dwight Whitney (1827–1894) and his intellectual descendants anticipated, and in part influenced, modern-day discussions of ‘lexical diffusion’, the piecemeal spread of phonological changes both through an individual speaker’s word-inventory and speaker-to-speaker through an entire community. Whitney regarded these two processes as inextricably linked; he also stressed comparison between different speech communities in search of locally distinct diffusionary patterns that challenged the Neogrammarian regularity hypothesis. Recent discussions of these and other aspects of lexical diffusion suggest that comparative-local studies, such as Whitney pioneered, remain essential for working out the explanatory nexus between lexical diffusion’s internally-phonological and social contexts.